Method and apparatus for coating fluorescent lamps



De 31, 1946- K. DEMB ET Al.

METHOD AND APPARATUS FOR COATYING FLUORESCENT LAMPS Filed Aug. l2, 1944 INVENToRs.

KENNETH f DEMB ,MALCOLM L. HAYES I ATTORNEY Patented Dec. 31,1946

UNITED STATE Y METHOD AND APPARATUS FOR COATING v FLUORESCENT LAMPS Kenneth Demb, Salem, andMalcolm L. Hayes,d Beverly, Mass., lassignors to Sylvania Electric Products Inc., Salem, Mass., a corporation of Massachusetts Application August 12, 1944, Serial No. 549,204

sciaims., (Crim-33.5)

This invention relates to a method and appa-v ratus for coatingthe inner surface of fluorescent `lamps with fluorescent material vand particularly yto such a method wherein the-coating material used has fast drying characteristics.

It is highly desirablethat the coating material upon fluorescent lamps be applied evenly and to a vpreek-:termined thickness. Heretoiore it has been 'considered of advantage to forego some of the advantages of a rapid drying coating in the interest of better quality obtainable, prior to the present invention, by using a slower drying solvent. The lslow drying solvent however, delayed thedrying time and the process showed poor production economy.

The practice of the presentinvention permits the use of rapid drying solvents, and produces a high quality coating in a greatly reduced period of time. I

The principles ofthe invention may be applied to many types of lamps but are particularly adapted to the coating of elongated open-ended lamp tubes. The coating material is prepared with ahighly volatile solvent such as acetone and is applied to the inner walls of the tubes by flowing it thereon at their upper ends while they are vertically disposed. The liquid coating flows downward wetting and coating the walls and the surplus liquid iiows out at the bottom Where it is collected to be used again.

The top of the, tubes. areleft openwhile the bottom openings are restrictedin their access to the atmosphere by controllable means. By such an arrangement certain natural properties of the vapors generated by the drying process are utilized to control and stabilize the drying conditions within the tubes in a manner to produce a perfect and highly eicient coating, as will appear more fully hereinafter.

An object of the invention is to provide means for rapidly coating fluorescent lamp envelopes with a'highly efficient light emitting coating.

A further object of the invention is to provide means for utilizing theadvantages of rapid drying solvents while controlling the evaporation conditions at `the coated surface in a manner to produce a uniform layer of fluorescent mate:` rial of predetermined thickness and density.

A further object of the invention is to provide a method of coating the inner surface of a lamp tube in which the natural vaporization characteristics of the solvents used in the coating material are employed to condition the' coating during the drying period.

Other objects and features will more fully ap- Figure j?. is across -out proper handling pear from the following description takenin connection with the accompanying drawing, and will belparticularly pointed out in the claims.

To'present a better understanding of 'the invention .a specific embodiment thereof is herein Vdescribed andv illustrated in the drawing in which:

Figure 1 is a diagrammatic view' of an apparatus suitable for practicing the invention;

Figure 2 is an'enlarged detail view', to more clearly vshow the structure of the tube'holding racks; and. p section on line 3'3,- Figure`2. M "A The desirability of using highly volatile solvents in the liquid material employed in coating fluorescent` lamps is well recognized. The art of 'sucfcessfully using such a material on a practical production scale however, had not been successfully developed prior to the present invention. Withand control, the resulting coating is inclined to be uneven and generally ineicient in light output. The coating material is prepared by mixing together'the desired 'iluorescent powder, a suitable temporary binder such as nitrocellulose, and a volatile solventthe major portion of which is acetone or other similarly volatile liquid. 1

The principles of the invention may be applied to the coatingfof many types of lamp envelopes.

However, the invention is herein applied -to the 'elongated type of lamp tubes which are open at Aboth ends when the coating isapplied,v The apparatus illustrated in the drawing enables the coating operation t be executed rapidly and accurately on a commercial scale. The lamp tubes 4 are held in a vertical position in racks 5 which are arranged to hold a plurality of tubes. A'posi, `tio'ning plate' is perforated with a number of holes 1 and is disposed' above a base plate 8 having apertures 9 therein situated in vertical alignment with the apertures 1. The tubes are loaded into the racks by passing them through the aperv turcs 1 andr projecting their end portions into the apertures 9.

Desirably the racks are so constructed that they may bemoved along a conveyor framework l lll. To feed the racks along the conveyor, a conveyor drive Il is provided having lugs IZ thereon which are positioned vto engage the racks 5 and move them along the ways lil. A suitable power '.m'eans and control not shown is provided which enables the operator to vposition the tubes successively beneath one'or more applicator nozzles I3 communicating with a supply of liquidv I3X.

The nozzles I3 are or may be of well-known construction having a hand or automatic means binder and the desired quantity of solvent is add-A y ed to give the material the correct consistency. The type of solvent used is important. It has been found that acetone alone is a satisfactory material to use but other highly volatile liquids may be used or other solvents in combination with acetone may also beused.

The proportion of uorescent powder temporary binder and solvent are adjusted to produce the required thickness of coating, when dried and baked.

As Vis wellk known, acetone .vaporizesi very rapidly at room temperature and its vapor is heavier than air. These properties are utilized to accomplish the desired end result. The whole operation of coating Aand drying is carried out at ordinary room temperature. Ii no control is exercised over the drying of the coating the rapid evaporation rate causes a sharp loweringof temperature at thel coatedzsurface. and a relatively violent down ow of the vapor results from its rapid generation. If the bottom of the tubes are open freely to the atmosphere the vapors rapidly pass downward and cause sudden drying of the lower portions of the coating prior to the drying of the upper portion, which, being liquid or semi-liquid tends to ow downward over the dried portions causing undesirable uneven dense areas. The drying period under such circumstances is very short but the whole operation produces a generally unsatisfactory result. If, however, the bottom opening of the tube is closed or partially closed to the flow of vapors a very diferent result is elected. By closing or partially closing the bottom end of the tubes and leaving the top ends open during the drying period a denite and highly benecial control is exercised over the drying operation. Under such conditions the solvent vapors as they are generated remain within thetube and in contact with the coating during the drying operation. The natural tendency. for the vapors is to ow downward since they are heavier than air. A small regulated portion only of the vapor is permitted to escape from the bottom of the tubes consequently a column of vapor automatically accumulates in the tube. Throughout the drying period therefore an atmosphere of vapo1` remains in the tube in contact with the coating. If an attempt is made to restrict the flow of vapor by any means other than at the bottom of the tubes a convection takes place at the tube bottom which causes premature drying at that point which results in an uneven coating. This effect is particularly noticeable in tubes of large diameter.

The process is carried out at normal room temperature and the natural tendency is for the solvent vapors to reach saturation pressure within the connes of the tube. Means are provided however to control the degree of vapor pressure vapor and air to escape. The escape of vapor and air at the bottom of the tube sets up a slow movement of air downward into the top. This air becomes saturated shortly after entering the top of the tube, after which it can no longer have, al drying eect. In the zone in whichl saturation is taking place the coating becomes dried. As this portion of the tube no longer contributes vapor to the stream of air, the portion immediately below it becomes the drying Zone.

. The-drying zone thus moves smoothly down the tube and the whole tube is dried progressively.

By varying4 the size of the adjustable opening during the drying period, automatically or manually, the density of coating along the length of the tube can be controlled, and it may be varied according to some predetermined pattern.

The control of the vapor at the base of the tubes Vmay be eiected in any suitable manner. In the construction illustrated a pan-like container I4 is secured to the bottom face of the base plate 8' of' the racks 5. The containers III serve to .enclose all of the openings 9 in the plate 8 and yalso tocatch the surplus liquid coating material which drains from the tubes. Adjustable openingsV IIE: are provided in the containers I4 by means of which the downward flow of vapor from the tubes may be regulated. The vapor iow Vfrom all vof the tubes in the rack is thus controlled by a single; opening. However, any suitable arrangement for accomplishing the same result may be employed.- As shown herein the size of the opening I5 may be adjusted by means of a sliding door I5, or other suitable means.

' The receptacles III are detachably secured to the racks in any desired manner such as by means of bolts I'I in the plate 8 having thumb nuts I8 thereon engaging the flange I9 upon the upper rimv of the receptacle. To produce a satisfactory vapor tight seal between the tubes and the apertures 9 suitable gaskets may be provided which are secured to the periphery of the apertures and have openings to tightly receive the tubes.V y

In brief the coating procedure may be reviewed as follows. The liquidy coating material is first prepared by mixing the desired type of iiuorescent powder, a suitable temporary binder such as nitrocellulose, and acetone, or similar highly volatile solvent. The proportions employed are calculated to provide the correct viscosity and Iiow characteristics required to produce a dried and baked coating of the desired density and light emission.

The coating liquid is placed in the supply conrainer ISX and racks 5 containing tubes to be coated are placed on the conveyor. The conveyor is then manipulated to bring one or more tubes into position under the nozzle or nozzles i3. The ends of the nozzles are inserted in the upper ends of the tubes and a quantity of coating liquid is nowed onto the inner walls of the tubes by operating the valve upon the nozzle I3. The liquid iiows downward wetting and depositing a coating on the tube walls, The surplus liquid drains into the receptacles I4. Immediately the coating starts to dry giving off heavy acetone fumes, some of which is permitted to escape through the opening I5. The opening is so regulated that the movement of air in the tube is restricted to the proper rate to provide the correct drying time for the coating. The tubes in the racks are successively coated and as they are moved along the conveyor become dried. yThe. whole procedure takes place at normal room temperature and Without forced air jets or vacuum devices. The vapor content of the exhaust air is therefore high allowing economical recovery of the solvent vapor by commonly known means.

The drainage liquid in the containers I4 is removed by detaching the containers and returning the liquid to the main supply or 'by a drainage tube.

In a short period of time the coating is dry and the tubes are removed and placed in a baking oven to remove the temporary binder after which they are ready to be received by the operator who performs the necessary operations to complete l the lamps.

By coating the lamps in accordance with the principles of the invention the total time consumed in coating and drying is greatly reduced from that required when slow drying solvents are used and moreover, due to the accurate control which is exercised over the drying conditions a more uniform and efcient coating is produced.

What we claim is:

1. A method of coating the inner surface of vertically disposed tubular uorescent lamp envelopes comprising the steps of preparing a liquid coating material containing fluorescent powder in which at least the major portion of its solvent is highly volatile and in which the solvent produces a vapor heavier than air, flowing the liquid onto the surface to be coated, draining and collecting the surplus coating liquidwhich passes completely through the tubes, restricting the downward iiow of solvent vapor and air within the tubes by partially closing the bottomends thereof where- 35 6 by the coating is dried in an atmosphere of solvent Vapor and air regulated to a predetermined rate of flow.

2. An apparatus for coating with iluorescent material the inner walls of open ended tubular lamp envelopes comprising a supply of liquid coating material having a solvent which produces a vapor heavier than air, an applicator nozzle connected with said supply for flowing the material upon the surface to be coated vapor tight means for collecting the surplus material which flows through thetubes and means in the wall of said collecting means acting to control the downward flow of vapor and air during the drying period to maintain the 'proper rate of drying within the tubes to produce an even coating of the required density.

fAnv apparatus for coating with uorescent materi'al'the inner walls of open ended tubular Vlamp envelopes comprising a supply of liquid coating material having a highly volatile solvent and in which the solvent produces a vapor heavier than air, an applicator nozzle connected to said supply operable to ow the material upon the surface to be coated, conveyor means for holding the tubes in upright position and positioning them beneath said nozzle, means for collecting the drainage from said tubes, and means for adjustably restricting the escape of solvent vapor and air from the bottom ends of the tubes during the drying period to restrict the rate of ow of air through the tubes to such a value that a smooth even coating of therequired density is produced.

KENNETH DEMB. MALCOLM L. HAYES. 

